At least five children were killed in the crash, school officials said, and six others are hospitalized in an intensive care unit.

"Five is a cursed number in our city right now. ... We are ... dealing with an unimaginable loss," Mayor Andy Berke said. "The most unnatural thing in the world is for a parent to mourn the loss of a child."

Woodmore Elementary School bus driver Johnthony Walker has been arrested and charged with multiple counts of vehicular homicide, reckless endangerment and reckless driving, according to Chattanooga Police Chief Fred Fletcher.

Authorities arrested the bus driver, charging him with vehicular homicide. But investigators are still working to pinpoint what caused the crash.

The bus driver, Johnthony Walker, 24, has been charged with five counts of vehicular homicide, reckless endangerment and reckless driving. He could face more charges as the case proceeds to a Hamilton County grand jury, Chattanooga police Chief Fred Fletcher said.

Walker has been cooperative and talking to investigators, police said.

Of the five children confirmed dead after Monday's crash, three were fourth-graders at Woodmore Elementary School, one was a first-grader and one was a kindergartner, according to Kirk Kelly, interim superintendent for Hamilton County Schools. Six students remain hospitalized in the intensive care unit, and six are in regular rooms at the hospital, Kelly said.

A spokeswoman from the Hamilton County district attorney's office earlier released a higher death toll, saying at least five children had died on the scene, and one had died at a hospital.

"There are still some unanswered questions at this time, but our priority remains with our students," Kelly said Tuesday morning, adding that counselors would be at the school for anyone who needs to talk.

"There are no words that you can say," Kelly said Tuesday morning. "This is something that you'll never get over ... but we're just doing what we can and reaching out and offering words of comfort and support to the families."

A woman who lives near the crash told WDEF the impact was so powerful it knocked her power out.

"I just heard a big boom," the woman said.

Speed is a possible factor

A National Transportation Safety Board team is expected to arrive in Chattanooga on Tuesday morning, Chairman Christopher Hart told reporters.

"Certainly, speed is being investigated very, very strongly as a factor in this crash," Fletcher said, adding that the investigation was still at an early stage.

He added that driving conditions were clear and dry. No other vehicles were involved in the crash.

Fletcher said "a warrant has been issued to remove the informational box on the school bus and review the video."

"Taking care of the children that are injured is our No. 1 priority. Taking care of the families who are looking for, caring for and grieving for children is our second priority," Fletcher said.

He added: "Our thoughts, our prayers and all of our efforts are with the families of these children and others impacted by this tragedy."

'Worst nightmare'

Fletcher called the crash site "a complicated crime scene" that covered a large area.

"What has happened today is every public safety official's absolute worst nightmare, but that is nothing in comparison to the nightmare that families and friends and our community is going through with this tragic loss to children in our community," Fletcher said.

A tweet from the Chattanooga Fire Department showed the bus on its side against a tree as emergency officials extricated the last passenger through the back door.

Sen. Bob Corker of Tennessee called the crash "heartbreaking" in a tweet.

'Hundreds and hundreds' of donors giving blood

Blood Assurance, a local blood bank, extended its hours at three locations as "hundreds and hundreds" of donors flocked to give blood to treat crash victims, spokeswoman Mindy Quinn said.

"We have had people who were refusing to leave until they give blood," Quinn said.

She said they were fast-tracking O-negative donors and trying to schedule donors for the days ahead and into next week.